In the known front attachment unit of Europatent document No. EP 00 18 667, the absorption coating has two layers. It consists of a metallic layer, which can be made inter alia from chromium or from a chromium alloy, but does not have to be made of these materials, and of a dielectric layer. The metallic layer has a complex index of refraction, in which the quotient of an imaginary and a real part lies in a range between 0.7 and 3.0. The dielectric layer has an index of refraction in the range of 1.35 to 1.70. By means of this double layer absorption coating, so-called halation is suppressed, i.e. the contrast is improved.
It is known from German patent DE No. 23 30 898 that an absorption coating arranged in the manner described can have the effect of increasing contrast. In the known construction, however, the contrast effect and reduction of reflection remain in need of improvement.